LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
10­
1
The
SWTR
and
Alternative
Disinfectants
and
Oxidants
Guidance
Manuals
are
available
on
EPA's
website,
http://
www.
epa.
gov/
safewater/
mdbp/
implement.
html.
10.0
Chlorine
Dioxide
10.1
Introduction
Chlorine
dioxide
is
used
for
disinfection,
taste
and
odor
control,
and
iron
and
manganese
removal.
Chlorine
dioxide
is
effective
for
inactivation
of
bacteria,
viruses,
and
protozoa,
including
Cryptosporidium
while
forming
fewer
halogenated
byproducts
than
chlorine.
It
is
stable
only
in
dilute
aqueous
solutions
and
must
be
generated
on­
site.
It
can
be
generated
using
a
variety
of
starting
materials
including
chloride,
chlorite,
or
chlorate.

The
Surface
Water
Treatment
Rule
(
SWTR)
and
subsequent
Stage
1
Disinfection
Byproducts
Rule
(
Stage
1
DBPR)
and
Interim
Enhanced
Surface
Water
Treatment
Rule
(
IESWTR)
all
recognize
the
ability
of
chlorine
dioxide
to
inactivate
pathogens.
As
a
result,
there
is
much
information
and
guidance
available
on
the
application
of
chlorine
dioxide
for
disinfection,
particularly
in
the
following
two
guidance
manuals:

°
Guidance
Manual
for
Compliance
with
the
Filtration
and
Disinfection
Requirements
for
Public
Water
Systems
Using
Surface
Water
Sources
(
USEPA
1991)
(
commonly
referred
to
as
the
Surface
Water
Treatment
Rule
Guidance
Manual).

 
Describes
how
to
calculate
the
CT
value
(
CT
is
described
in
the
next
sub­
section)
for
a
given
disinfectant,
including
methodologies
for
determining
the
residual
concentration
(
C)
and
contact
time
(
T).

 
Includes
CT
values
for
log­
inactivation
of
Giardia
and
viruses.

°
Alternative
Disinfectants
and
Oxidants
Guidance
Manual
(
USEPA
1999).

 
Provides
full
descriptions
of:
°
chlorine
dioxide
chemistry
°
on­
site
generation
°
primary
uses
and
points
of
applications
°
pathogen
inactivation
and
disinfection
efficiency
°
byproduct
production
°
analytical
methods
°
operational
considerations
The
purpose
of
this
chapter
is
to
(
1)
describe
what
systems
need
to
do
to
achieve
Cryptosporidium
inactivation
treatment
credit
for
disinfecting
with
chlorine
dioxide,
(
2)
discuss
Chapter
10
­
Chlorine
Dioxide
LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
10­
2
design
and
operational
considerations
that
will
assist
water
systems
in
deciding
whether
this
toolbox
option
is
a
practical
option
for
them,
and
(
3)
discuss
key
issues
associated
with
using
chlorine
dioxide
as
a
disinfectant.
This
chapter
is
organized
as
follows:

10.2
Log
Inactivation
Requirements
­
describes
the
concentration
and
time
variables
of
the
CT
parameter,
presents
the
chlorine
dioxide
CT
table
for
Cryptosporidium,
and
provides
a
sample
CT
calculation.

10.3
Monitoring
Requirements
­
describes
monitoring
requirements
of
both
LT2ESWTR
and
Stage
1
DBPR.

10.4
Unfiltered
Systems
LT2ESWTR
Requirements
­
describes
the
level
of
Cryptosporidium
inactivation
unfiltered
systems
must
provide,
and
monitoring
requirements
that
must
be
met.

10.5
Disinfection
with
Chlorine
Dioxide
­
describes
chlorine
dioxide
chemistry
and
disinfection
with
chlorine
dioxide.

10.6
Toolbox
Selection
Considerations
­
discusses
the
advantages
and
disadvantages
of
disinfection
with
chlorine
dioxide.

10.7
Design
Considerations
­
discusses
effects
of
temperature
and
the
point
of
chlorine
dioxide
addition
on
achieving
the
required
CT
value.

10.8
Operational
Considerations
­
discusses
water
quality
parameters
that
affect
the
disinfection
ability
of
chlorine
dioxide.

10.9
Safety
Issues
­
describes
considerations
for
chemical
storage
and
discusses
the
acute
health
risks
of
chlorine
dioxide.

10.2
Log
Inactivation
Requirements
Systems
can
achieve
anywhere
from
0.5
to
3.0
log
Cryptosporidium
inactivation
with
the
addition
of
chlorine
dioxide.
The
amount
of
Cryptosporidium
inactivation
credit
a
system
may
receive
is
determined
by
the
CT
provided
in
the
treatment
process
(
40
CFR
141.729(
b)).
This
methodology
provides
a
conservative
characterization
of
the
dose
of
chlorine
dioxide
necessary
to
achieve
a
specified
inactivation
level
of
Cryptosporidium.
CT
is
the
product
of
the
disinfectant
concentration
and
disinfectant
contact
time
and
is
defined
in
the
LT2ESWTR
(
40
CFR
141.729(
a)):

CT
=
Disinfectant
(
mg/
L)
x
Contact
Time
(
minutes)
Chapter
10
­
Chlorine
Dioxide
LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
10­
3
°
"
T"
is
the
time
(
in
minutes)
it
takes
the
water,
during
peak
hourly
flow,
to
move
from
the
point
of
disinfectant
application
to
a
point
where,
C,
residual
concentration
is
measured
prior
to
the
first
customer,
or
between
points
of
residual
measurement.

°
"
C"
is
the
concentration
of
chlorine
dioxide
present
in
the
system,
expressed
in
mg/
L.

The
concept
of
regulating
surface
water
treatment
disinfection
processes
through
CT
was
first
introduced
in
the
SWTR.
Tables
of
Giardia
and
virus
log
inactivations
correlated
to
CT
values,
commonly
referred
to
as
CT
tables,
were
presented
in
the
SWTR
Guidance
Manual.
For
the
LT2ESWTR,
EPA
developed
CT
tables
for
the
inactivation
of
Cryptosporidium.
Alternatively,
a
system
may
conduct
a
site­
specific
study
to
determine
the
CT
values
necessary
to
meet
a
specified
log
inactivation,
using
State
approval
(
40
CFR
141.729(
b)(
4)).
Appendix
A
provides
guidance
for
conducting
a
site­
specific
study.

10.2.1
CT
Calculation
The
methodology
and
calculations
for
determining
CT
have
not
changed
from
the
SWTR
to
the
LT2ESWTR
requirements.
This
section
briefly
reviews
how
CT
is
used
to
determine
loginactivation
for
the
SWTR
and
presents
the
chlorine
dioxide
CT
table
for
Cryptosporidium
inactivation.
Refer
to
the
SWTR
Guidance
Manual
for
descriptions
of
measuring
C
and
determining
T.

Summary
of
CT
Determination
and
Corresponding
Log­
inactivation
as
Presented
in
the
SWTR
Guidance
Manual
CT
can
be
calculated
for
an
entire
treatment
process
or
broken
into
segments
and
summed
for
a
total
CT
value.
C
is
measured
at
the
end
of
a
given
segment.
T
is
generally
estimated
by
methods
involving
established
criteria
(
flow,
volume,
and
contactor
geometry)
or
tracer
studies.
The
following
steps
describe
the
CT
calculation
from
measured
C
and
T
values
for
a
segment
of
the
entire
treatment
process:

1)
Calculate
CT
calc
by
multiplying
the
measured
C
and
T
values.

2)
From
the
CT
tables,
find
the
CT
value
for
the
log
inactivation
desired,
this
is
CT
table.

3)
Calculate
the
ratio
of
CT
calc/
CT
table
for
each
segment.

4)
If
a
system
has
multiple
segments,
sum
the
CT
calc/
CT
table
ratios
for
a
total
inactivation
ratio.
Chapter
10
­
Chlorine
Dioxide
LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
10­
4
5)
If
the
ratio
of
CT
calc/
CT
table
is
at
least
1,
then
the
treatment
process
provides
the
log
inactivation
that
the
CT
table
represents
(
log
inactivation
desired
from
step
#
2).

Table
10.1
CT
Values
(
mg­
min/
l)
for
Cryptosporidium
Inactivation
by
ClO
2
Log
credit
Water
Temperature,

C1
<=
0.5
1
2
3
5
7
10
15
20
25
0.5
319
305
279
256
214
180
138
89
58
38
1.0
637
610
558
511
429
360
277
179
116
75
1.5
956
915
838
767
643
539
415
268
174
113
2.0
1275
1220
1117
1023
858
719
553
357
232
150
2.5
1594
1525
1396
1278
1072
899
691
447
289
188
3.0
1912
1830
1675
1534
1286
1079
830
536
347
226
1CT
values
between
the
indicated
temperatures
may
be
determined
by
interpolation
Example
CT
Calculation
A
plant
draws
1.5
MGD
of
5
degrees
Celsius
water
from
a
stream,
adding
1.8
mg/
l
of
chlorine
dioxide
at
the
intake.
The
water
travels
through
2
miles
of
12
inch
pipe
to
a
settling
tank.
The
detention
time
in
the
tank,
as
determined
by
a
tracer
study,
is
150
minutes.
After
the
tank,
it
travels
through
another
12­
inch
pipe
to
the
plant.
Figure
10.1
provides
a
schematic
of
an
intake,
piping,
and
tank.
The
concentration
of
chlorine
dioxide
at
each
point
is
measured
as
follows:

C
initial
=
1.8
mg/
l
C
entering
tank
=
1.6
mg/
l
C
leaving
tank
=
0.8
mg/
l
C
leaving
2nd
pipe
=
0.2
mg/
l
Chapter
10
­
Chlorine
Dioxide
LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
10­
5
C
in
=
1.8
mg/
l
2
miles
0.25
miles
Segment
1
Segment
2
Segment
3
Transmission
Line
Sumdac
River
Big
Tank
Figure
10.1
CT
Calculation
Example
Schematic
The
residence
times
of
the
two
sections
of
pipe
are
determined
assuming
plug
flow.
Therefore
the
time
for
each
section
is
calculated
as
follows:

T
1
=
(
A
1*
L
1/
Q
1)
=
(
 r2L
1/
Q
1)*(
7.48
gal./
1
ft.
3)*(
MG/
1,000,000
gal.)*(
1,440
min./
day)

where:

$
A
is
the
cross­
sectional
area
of
the
pipe
in
square
feet
$
Q
is
the
volumetric
flow
rate
in
MGD
$
L
is
the
length
of
pipe
in
feet
$
r
is
the
radius
of
the
pipe
in
feet.

Therefore
the
times
for
the
two
sections
of
the
pipe
are
as
follows:

T
1
=
2
mi.*(
5,280
ft./
mi.)*
 *(
0.5
ft.)
2*(
0.0108
MG*
sec/
ft.
3*
day)/(
1.5
MGD)
=
59.7
min.
T
3
=
0.25
mi.*(
5,280
ft./
mi.)*
 *(
0.5
ft.)
2*(
0.0108
MG*
sec/
ft.
3*
day)/(
1.5
MGD)
=
7.4
min.

The
T
10,
or
time
for
90
percent
of
a
tracer
to
pass
through
the
section
for
the
tank
is
as
follows:

T
2
=
150
minutes
Chapter
10
­
Chlorine
Dioxide
LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
10­
6
CT
Calculation:

Step
1.
Calculate
CT
for
each
segment.

The
concentrations
and
times
for
each
segment
are
known.
The
T's
are
calculated
above
and
the
C
is
the
concentration
measured
at
the
end
of
each
segment.
The
CT
for
each
segment
is
calculated
as
follows:

CT
1
=
(
1.6
mg/
l)


(
59.5
min.)
=
95.2
mg



min./
l
CT
2
=
(
0.8
mg/
l)


(
150
min.)
=
120
mg


min./
l
CT
3
=
(
0.2
mg/
l)


(
7.4
min.)
=
1.5
mg


min./
l
Step
2.
Look
up
CT
table
in
Table
10.1.
For
5oC
and
0.5
log
inactivation,

CT
table
=
214
mg

min./
l.

Step
3.
Calculate
the
ratio
of
CT
calc/
CT
table
for
each
segment.

(
CT
calc/
CT
table)
1
=
95.2/
214
=
0.44
(
CT
calc/
CT
table)
2
=
120
/
214
=
0.56
(
CT
calc/
CT
table)
3
=
1.5/
214
=
0.01
Step
4.
Sum
the
CT
calc/
CT
table
for
each
segment.

(
CT
calc/
CT
table)
total
=
0.44
+
0.56
+
0.01
=
1.01
Determine
Log
Inactivation:

If
the
result
of
Step
4
is
greater
than
1,
the
log
inactivation
associated
with
the
CT
table
values
is
achieved.
If
the
result
is
less
than
1,
that
level
of
log
inactivation
is
not
achieved
(
if
the
log
inactivation
was
less
than
1.0,
the
calculations
should
be
repeated
at
a
lower
log
inactivation).
In
this
example,
the
sum
of
the
CT
calc/
CT
table
for
all
the
segments
is
greater
than
1,
so
the
system
qualifies
for
a
0.5
log
Cryptosporidium
inactivation.

10.3
Monitoring
Requirements
10.3.1
LT2ESWTR
The
LT2ESWTR
requires
daily
CT
monitoring
(
40
CFR
141.730),
which
must
be
done
during
peak
hourly
flow.
Since
systems
may
not
know
when
the
peak
hourly
flow
will
occur,
EPA
recommends
monitoring
on
an
hourly
basis.
Contact
time
does
not
have
to
be
determined
Chapter
10
­
Chlorine
Dioxide
LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
10­
7
on
a
daily
basis,
only
concentration
does.
Contact
time
is
determined
using
the
peak
hourly
flow.
Systems
should
reevaluate
contact
time
whenever
they
modify
a
process
and
the
hydraulics
are
affected
(
e.
g.,
add
a
pump
for
increased
flow,
reconfigure
piping).

The
chlorine
dioxide
concentration
should
be
measured
using
approved
analytical
methods,
either
DPD,
(
Standard
Method
4500­
ClO
2
D)
or
Amperometric
Method
II,
(
Standard
Method
4500­
ClO
2
E).
Details
on
these
methods
can
be
found
in
Standard
Methods
for
the
Examination
of
Water
and
Wastewater,
20th
edition,
American
Public
Health
Association,
1998.

Note,
if
a
system
is
required
to
develop
a
disinfection
profile
under
the
LT2ESWTR
and
changes
its
disinfection
process,
the
LT2ESWTR
requires
the
system
to
calculate
a
disinfection
profile
and
benchmark
(
40
CFR
141.714(
a))
(
see
Chapter
1,
section
1.6
for
details).

10.3.2
Stage
1
DBPR
The
Stage
1
DBPR
requires
all
systems
using
chlorine
dioxide
for
disinfection
or
oxidation
to
monitor
daily
for
chlorine
dioxide
and
chlorite
at
the
distribution
system
entry
point.
In
addition,
systems
must
take
monthly
chlorite
samples
at
three
locations
in
the
distribution
system.
Table
10.2
lists
the
chlorine
dioxide
and
chlorite
distribution
system
monitoring
requirements.

Table
10.2
Distribution
System
Monitoring
Requirements
at
Each
Plant
Location
Frequency
Chlorite
Distribution
System
Entry
Point
Daily
Distribution
System
Sample
Set
of
3:
1
Near
First
Customer
1
In
Middle
of
the
Distribution
System
1
At
Maximum
Residence
Time
Monthly
Chlorine
Dioxide
Distribution
System
Entry
Point
Daily
If
the
chlorine
dioxide
maximum
residual
disinfectant
level
(
MRDL)
of
0.8
mg/
L
or
the
chlorite
maximum
contaminant
level
(
MCL)
of
1.0
mg/
L
is
exceeded
in
any
of
the
samples,
additional
monitoring
is
required
(
see
the
Stage
1
DBPR,
40
CFR141.132(
b)
for
further
information).
The
monthly
monitoring
requirements
for
chlorite
may
be
reduced
if
all
chlorite
samples
are
below
the
MCL
for
a
1­
year
period.
Chapter
10
­
Chlorine
Dioxide
LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
10­
8
10.4
Unfiltered
System
LT2ESWTR
Requirements
The
LT2ESWTR
requires
unfiltered
systems
to
provide
at
least
2.0
log
Cryptosporidium
inactivation
(
40
CFR
141.721(
b)).
If
their
source
water
Cryptosporidium
concentration
is
greater
than
0.01
oocyst/
liter,
then
systems
must
provide
3.0
log
Cryptosporidium
inactivation
(
40
CFR
141.721(
b)).
The
requirements
of
the
previous
SWTR
regulations
still
apply
 
achieve
3
log
inactivation
of
Giardia
and
4
log
inactivation
of
viruses
and
maintain
a
disinfectant
residual
in
the
distribution
system
(
e.
g.,
free
chlorine
or
chloramines).
LT2ESWTR
also
requires
that
a
minimum
of
two
disinfectants
be
used
to
meet
overall
disinfection
requirements.

The
monitoring
requirements
described
in
section
10.3
apply
to
unfiltered
systems.
Additionally,
the
LT2ESWTR
requires
unfiltered
systems
to
meet
the
Cryptosporidium
loginactivation
requirements
determined
by
the
daily
CT
value
every
day
the
system
serves
water
to
the
public,
except
one
day
per
calendar
month
(
40
CFR
141.721(
c)).
Therefore,
if
an
unfiltered
system
fails
to
meet
Cryptosporidium
log­
inactivation
two
days
in
a
month,
it
is
in
violation
of
the
treatment
technique
requirement.

10.5
Disinfection
With
Chlorine
Dioxide
Chlorine
dioxide
(
ClO
2)
is
an
uncharged
compound
of
chlorine
in
the
+
IV
oxidation
state.
It
is
a
relatively
small,
volatile,
and
highly
energetic
molecule,
and
a
free
radical
even
in
dilute
aqueous
solutions.
At
high
concentrations,
it
reacts
violently
with
reducing
agents.
However,
it
is
stable
in
dilute
solution
in
a
closed
container
in
the
absence
of
light.
When
an
aqueous
solution
is
open
to
the
atmosphere,
chlorine
dioxide
readily
comes
out
of
solution.
Aqueous
solutions
of
chlorine
dioxide
are
also
susceptible
to
photolytic
decomposition,
depending
on
the
time
of
exposure
and
intensity
of
UV
light.

Disinfection
of
protozoa
is
believed
to
occur
by
oxidation
reactions
disrupting
the
permeability
of
the
cell
wall
(
Aieta
and
Berg
1986).
Chlorine
dioxide
functions
as
a
highly
selective
oxidant
due
to
its
unique,
one­
electron
transfer
mechanism
where
it
is
reduced
to
chlorite
(
ClO
2
­)
(
Hoehn
et
al.
1996).

In
drinking
water,
chlorite
(
ClO
2
­)
is
the
predominant
reaction
end
product,
with
approximately
50
to
70
percent
of
the
chlorine
dioxide
converted
to
chlorite
and
30
percent
to
chlorate
(
ClO
3
­)
and
chloride
(
Cl­)
(
Werdehoff
and
Singer
1987).
This
has
a
significant
impact
on
disinfection
capabilities
for
drinking
water,
since
chlorite
is
a
regulated
drinking
water
contaminant
with
an
MCL
of
1.0
mg/
L.
Based
on
a
50
to
70
percent
conversion
of
chlorine
dioxide
to
chlorite,
the
maximum
dose
is
limited
to
1.4
to
2.0
mg/
l
unless
the
chlorite
is
removed
through
subsequent
treatment
processes.
Chapter
10
­
Chlorine
Dioxide
LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
10­
9
10.6
Toolbox
Selection
Considerations
10.6.1
Advantages
There
are
several
advantages
to
using
chlorine
dioxide
as
a
primary
disinfectant.
Chlorine
dioxide
is
approximately
four
times
as
effective
as
chlorine
for
the
inactivation
of
Giardia
and
is
a
stronger
disinfectant
than
chlorine
for
bacteria
(
White
1999).
However,
free
chlorine
is
more
effective
for
the
inactivation
of
viruses.
Other
advantages
of
disinfection
with
chlorine
dioxide
include:

°
A
high
oxidizing
potential
allows
it
to
oxidize
other
compounds
such
as
manganese
and
some
taste
and
odor
compounds.

°
Chlorine
dioxide
does
not
form
regulated
halogenated
organic
byproducts.

°
The
effect
of
pH
on
the
disinfection
ability
of
chlorine
dioxide
is
much
smaller
than
for
other
disinfectants.

°
Chlorine
dioxide
has
shown
a
synergistic
effect
when
combined
with
other
disinfectants
such
as
ozone,
chlorine,
and
chloramines
that
leads
to
greater
inactivation
with
the
disinfectants
added
in
series
than
by
either
disinfectant
individually.

10.6.2
Disadvantages
A
major
disadvantage
of
chlorine
dioxide
is
the
byproduct
formation
of
chlorite
and
chlorate.
Section
10.6
describes
the
dose
limits
of
chlorine
dioxide
due
to
the
formation
of
chlorite.
Other
disadvantages
of
disinfection
with
chlorine
dioxide
include:

°
Difficulty
in
maintaining
an
effective
residual.
Additionally,
residual
will
be
lost
in
the
filters.

°
It
decomposes
upon
exposure
to
sunlight,
flourescent
light
bulbs,
and
UV
disinfection
systems.

°
Ability
to
disinfect
is
reduced
under
colder
temperatures.

°
If
the
ratio
of
reactants
in
the
chlorine
dioxide
generator
is
incorrect,
excess
aqueous
chlorine
can
remain,
which
can
form
halogenated
disinfection
byproducts.

°
Chlorine
dioxide
must
be
generated
on­
site.
Chapter
10
­
Chlorine
Dioxide
LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
10­
10
°
There
may
be
a
need
for
three­
phase
power
which
may
not
be
compatible
with
some
water
systems.

°
Chlorine
dioxide
can
be
explosive
at
high
temperatures
or
pressures.

°
Storage
of
sodium
chlorite
solution
can
be
problematic
due
to
crystallization
at
low
temperatures
or
high
concentrations
and
stratification
at
temperatures
below
40"
F
(
or
4"
C).

°
High
cost
of
chemicals.

°
Dialysis
patients
are
sensitive
to
higher
chlorite
levels
and
should
be
notified
if
chlorine
dioxide
is
going
to
be
added
where
it
has
not
routinely
been
used.

°
Training,
sampling,
and
analysis
costs
are
high.

Systems
considering
using
chlorine
dioxide
as
a
disinfectant
should
perform
chlorine
dioxide
demand/
decay
tests
on
the
water
being
considered
for
disinfection
(
raw
water
or
filter
effluent)
under
normal
and
poor
water
quality
conditions.
If
chlorine
dioxide
is
added
where
the
demand
is
1.4
mg/
l
or
greater,
the
system
may
have
difficulty
complying
with
the
chlorite
MCL.
If
the
raw
water
has
a
chlorine
dioxide
requirement
greater
than
1.4
mg/
l,
chlorine
dioxide
might
still
be
able
to
be
used
for
post
disinfection
since
the
oxidant
demand
will
be
less
after
the
filters.

10.7
Design
Considerations
10.7.1
Designing
to
Lowest
Temperature
As
the
water
temperature
declines,
chlorine
dioxide
becomes
less
effective
as
a
disinfectant.
LeChevallier
et
al.
(
1997)
found
that
reducing
the
temperature
from
20
degrees
Celsius
to
10
degrees
Celsius
reduced
disinfection
effectiveness
by
40
percent.
Since
the
treatment
achieved
for
chlorine
dioxide
addition
is
temperature
dependent,
systems
need
to
consider
the
variability
in
water
temperature
to
ensure
they
meet
the
CT
level
for
the
minimum
treatment
needed
for
compliance.
For
example,
if
a
system
is
required
to
provide
an
additional
1
log
Cryptosporidium
treatment
and
plans
to
achieve
that
with
chlorine
dioxide
alone,
then
it
should
determine
the
CT
required
for
the
lowest
water
temperature
experienced
and
ensure
it
can
meet
those
CT
requirements.
Chapter
10
­
Chlorine
Dioxide
LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
10­
11
10.7.2
Point
of
Addition
There
are
two
main
considerations
for
determining
locations
of
chlorine
dioxide
addition
for
the
purpose
of
Cryptosporidium
inactivation
 
contact
time
and
chlorine
dioxide
demand.
Additionally,
systems
using
ozone
should
consider
that
ozone
will
degrade
chlorine
dioxide.
The
application
point
for
chlorine
dioxide
should
be
well
upstream
of
the
ozone
process
or
just
after
the
ozone
process.

Contact
Time
There
must
be
substantial
contact
time
with
a
residual
concentration.
The
CT
requirements
for
Cryptosporidium
are
much
higher
than
for
Giardia
and
viruses
and
when
designing
to
the
lowest
water
temperatures,
the
resulting
contact
time
requirements
are
relatively
high
for
even
the
0.5
and
1.0
log
inactivation.
Chlorine
dioxide
readily
degrades
when
exposed
to
light
from
flourescent
lamps
or
the
sun,
therefore
all
the
available
concentration
in
open
basins
will
most
likely
not
be
utilized
for
disinfection.
For
most
systems,
the
point
of
application
will
be
either
at
the
raw
water
intake
or
after
the
filters,
whichever
can
provide
the
necessary
contact
time.

Oxidant
Demand
The
oxidant
demand
of
the
water
affects
chlorite
and
chlorate
byproduct
formation
(
section
10.6).
If
the
chlorine
dioxide
requirement
of
the
raw
water
is
greater
than
1.4
mg/
L
then
chlorite
concentration
will
likely
exceed
the
MCL.
However,
chlorine
dioxide
could
be
added
after
the
filters
where
the
oxidant
demand
is
frequently
lower
and,
therefore,
a
lower
dose
of
chlorine
dioxide
would
result
in
a
lower
byproduct
concentration
of
chlorite.

10.8
Operational
Considerations
Of
all
the
water
quality
parameters,
water
temperature
has
the
strongest
effect
on
the
disinfection
ability
of
chlorine
dioxide.
The
concentration
of
suspended
matter
and
pH
also
have
an
effect,
but
to
a
lesser
extent
than
temperature.
Although
the
disinfection
potential
of
chlorine
dioxide
is
not
strongly
affected
by
pH,
studies
have
shown
that
chlorine
dioxide
disinfection
is
better
under
higher
pH
(
LeChevallier
et
al.
1997).

Suspended
matter
and
pathogen
aggregation
affect
the
disinfection
efficiency
of
chlorine
dioxide.
Protection
from
chlorine
dioxide
inactivation
due
to
bentonite
was
determined
to
be
approximately
11
percent
for
water
with
turbidity
values
less
than
or
equal
to
5
NTU
and
25
percent
for
turbidity
between
5
and
17
NTUs
(
Chen
et
al.
1984).
Chapter
10
­
Chlorine
Dioxide
LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
10­
12
Airborne
concentrations
greater
than
10
percent
may
cause
explosions.
Based
on
the
research
discussed
above,
the
optimal
conditions
for
Cryptosporidium
disinfection
with
chlorine
dioxide
are
low
turbidity,
high
pH,
and
high
temperature.

10.9
Safety
Issues
Because
chlorine
dioxide
can
be
explosive
and
pose
acute
health
risks
to
those
exposed
to
gaseous
chlorine
dioxide,
a
safety
plan
should
be
developed
that
includes
precautions
for
generation,
handling,
storage,
and
emergency
response.

10.9.1
Chemical
Storage
Most
chlorine
dioxide
generators
use
sodium
chlorite
solutions
as
a
raw
material.
If
sodium
chlorite
solutions
are
accidently
acidified
or
exposed
to
a
reducing
agent,
uncontrolled
production
and
release
of
gaseous
chlorine
dioxide
can
result.
In
addition
to
being
toxic,
if
the
gaseous
chlorine
dioxide
reaches
concentrations
greater
than
10
percent,
it
can
spontaneously
explode.

Sodium
chlorite
should
be
stored
away
from
other
chemicals,
particularly
any
acid
solutions
or
chemicals
that
could
act
as
reducing
agents.
Construction
materials
in
sodium
chlorite
storage
areas,
as
well
as
chlorine
dioxide
generating
areas,
should
be
fire
resistant
such
as
concrete.
Sodium
chlorite
fires
burn
especially
hot
and
produce
oxygen
as
a
byproduct,
so
special
fire
fighting
techniques
are
required
to
extinguish
the
fire.
These
firefighting
techniques
should
be
part
of
the
safety
plan
and
proper
equipment
and
supplies
should
be
stored
nearby.
Temperatures
in
storage
and
generation
areas
should
be
kept
below
130
degrees
Celsius.

10.9.2
Acute
Health
Risks
of
Chlorine
Dioxide
Exposure
to
gaseous
chlorine
dioxide
can
cause
shortness
of
breath,
coughing,
respiratory
distress,
and
pulmonary
edema.
The
Occupational
Safety
and
Health
Administration
(
OSHA)
permissible
exposure
limit
(
PEL)
is
0.1
ppm.
Areas
where
chlorine
dioxide
is
generated
and
stored
should
have
appropriate
monitoring
to
detect
leaks
of
chlorine
dioxide
or
other
chlorine
containing
chemicals
into
the
air.
Proper
ventilation
and
scrubbing
systems
should
be
installed.
First
aid
kits
and
respirators
should
also
be
accessible
outside
the
building.
Operators
should
be
trained
to
use
the
respirators.
Chapter
10
­
Chlorine
Dioxide
LT2ESWTR
Toolbox
Guidance
Manual
Proposal
Draft
June
2003
10­
13
References
APHA.
1998.
Standard
Methods
for
the
Examination
of
Water
and
Wastewater,
20th
edition,
American
Public
Health
Association.

Aieta,
E.,
and
J.
D.
Berg.
1986.
"
A
Review
of
Chlorine
Dioxide
in
Drinking
Water
Treatment."
J.
AWWA.
78(
6):
62­
72.

Chen,
Y.
S.
R.,
O.
J.
Sproul,
and
A.
J.
Rubin.
1984.
"
Inactivation
of
Naegleria
Gruberi
cysts
by
Chlorine
Dioxide."
EPA
Grant
R808150­
02­
0,
Department
of
Civil
Engineering,
Ohio
State
University.

Hoehn,
R.
C.,
A.
A.
Rosenblatt,
and
D.
J.
Gates.
1996.
"
Considerations
for
Chlorine
Dioxide
Treatment
of
Drinking
Water."
Conference
proceedings,
AWWA
Water
Quality
Technology
Conference,
Boston,
MA.

LeChevallier,
M.
W.,
et
al.
1997.
"
Chlorine
Dioxide
for
Control
of
Cryptosporidium
and
Disinfection
Byproducts."
Conference
proceedings,
1996
AWWA
Water
Quality
Technology
Conference
Part
II,
Boston,
Massachusetts.

USEPA
1999.
Alternative
Disinfectants
and
Oxidants
Guidance
Manual.
Washington,
D.
C.

USEPA,
1991.
Guidance
Manual
for
Compliance
with
the
Filtration
and
Disinfection
Requirements
for
Public
Water
Systems
Using
Surface
Water
Sources.
Washington,
D.
C.

Werdehoff,
K.
S,
and
P.
C.
Singer.
1987.
"
Chlorine
Dioxide
Effects
on
THMFP,
TOXFP
and
the
Formation
of
Inorganic
By­
Products."
J.
AWWA.
79(
9):
107.

White,
Geo.
Clifford.
1999.
Handbook
of
Chlorination
and
Alternative
Disinfectants,
4th
edition,
John
Wiley
&
Sons,
Inc.
